In many optical articles including various displays such as computer displays, TV and plasma displays, liquid crystal display devices, transparent plastic lenses, covers in various instruments, and window panes in automobiles and trains, antireflective coatings are often used as an outermost layer for the purpose of improving visibility. The principle of antireflection requires that the antireflective coating have a low refractive index.
Fluoroplastics are employed in the antireflection application as in displays because they essentially have a low refractive index and satisfactory alkali resistance. However, since the fluoroplastics are often used as rubber due to their molecular structure, it is difficult to form hard protective coatings having mar resistance from such fluoroplastics.
Recently, hydrolyzable silane compounds having perfluoroalkyl groups were developed. To take advantage of their favorable characteristics, a variety of coating compositions comprising hydrolyzable silane compounds have been developed for imparting alkali resistance, water repellency, oil repellency, anti-staining and anti-reflection. However, since perfluoroalkyl groups contributing to these favorable characteristics are bulky and inert, the cured coatings have a low crosslink density. As a result, the cured coatings are fairly hard as compared with fluoroplastics, but still lack mar resistance.
Several systems were developed for enhanced mar resistance. For example, JP-A 2002-53805 describes co-hydrolysis of a perfluoroalkyl-containing silane and a silane compound such as a tetraalkoxysilane; JP-A 61-40845 and JP-B 6-29332 describe systems using a bis-silane compound having perfluoroalkylene as a spacer in combination with a perfluoroalkyl-containing silane and a tetraalkoxysilane or silane coupling agent for imparting both mar resistance and anti-staining properties. The system using tetraalkoxysilane is undesirable in that the Q unit moiety is so weak to alkali that the coatings can be deteriorated when they are washed with strong alkaline detergents for household use. Also, Japanese Patent No. 2,629,813 describes a system comprising a bis-silane compound having perfluoroalkylene as a spacer and an epoxy-functional silane. This system achieves fairly satisfactory levels of desired properties such as anti-staining, mar resistance, adhesion and antireflection. However, it lacks alkali resistance because of a reduced fluorine content and the hydrophilicity of epoxy groups. This is problematic on practical use.
In one example described in the above Japanese Patent No. 2,629,813, a combination of a bis-silane compound having perfluoroalkylene as a spacer and a perfluoroalkyl-containing silane is applied to a transparent plastic substrate having a scattering irregular surface. The perfluoroalkylene group of the bis-silane compound used therein has a long chain length, which suggests a long distance between crosslinking sites in the resulting cured coating with the accompanying drawback of failing to provide sufficient mar resistance. No reference is made to alkali resistance. The essential characteristics to meet satisfactory alkali resistance were then not acknowledged. Japanese Patent No. 3,210,045 describes a similar system in which alkali resistance on hard glass substrates is evaluated. Examples use a test of evaluating alkali resistance by immersing a sample wholly in a 1% NaOH aqueous solution for 24 hours, washing the sample immediately thereafter, and inspecting the sample for outer appearance change and water droplet removal. The coats examined therein include two types of coats, a coat of perfluoroalkyl-containing silane alone and a coat of a ternary system comprising a bis-silane compound having perfluoroalkylene as a spacer, a perfluoroalkyl-containing silane, and a tetraalkoxysilane. The present inventor performed evaluation by another test of placing a droplet of alkaline solution on a coat, wiping off the droplet after 30 minutes, and visually inspecting for any outer appearance change. In the ternary system coat, the area where a droplet had been rested became slightly white hazed and was distinguishable in outer appearance from the colorless transparent area where no droplet was rested, indicating insufficient alkali resistance. The test of immersing a sample wholly in an alkaline solution is an insufficient evaluation in that since the entire sample is uniformly deteriorated, any change of outer appearance is not easily perceivable. Many of the samples which are evaluated as passing the whole immersion test seem to have insufficient alkali resistance on practical use. The coat of perfluoroalkyl-containing silane alone, when applied to synthetic resin substrates, is less resistant to marring as mentioned above. As discussed above, none of the known systems have recognized the requirements to meet all of alkali resistance, mar resistance and anti-staining properties that the present inventor has found.
There are available no antireflective coating compositions that possess multiple functions of chemical resistance (inter alia, alkali resistance), mar resistance, and anti-staining.